1. Field of the Invention
The present invention relates to a comparator with an offset canceling function and D/A conversion apparatus with an offset canceling function.
2. Description of the Related Art
A CDMA transmitter converts QPSK-modulated I (in-phase) and Q (quadrature) signals from digital to analog by a D/A converter and sends these signals to an antenna.
An input/output characteristic of the D/A converter (relationship between an input digital value and analog output voltage) should be ideally equal for all D/A converters.
However, an input/output offset is actually produced due to various factors.
When an amount of this offset differs between the D/A converters corresponding to the I signal and Q signal (that is, the two D/A converters have different input/output characteristics), the phases of the I and Q signals are shifted, which results in a transmission error.
Therefore, it is necessary to cancel the input/output offset of the D/A converters so that both D/A converters have a same characteristic.
Canceling the input/output offset of the D/A converters first requires the amount of offset to be measured and a comparator (voltage comparator) is used for this purpose.
Based on the comparison result of this comparator, a control signal to compensate for the offset is obtained to cancel the input/output offset of the D/A converters.
A configuration for canceling such an input/output offset of the D/A converters is described, for example, in the Unexamined Japanese Patent Publication No.HEI 11-234130.
As described above, it is necessary to measure the amount of offset using a comparator in order to cancel the input/output offset of the D/A converters.
For measurement of a correct amount of offset, it is naturally important that the offset of the comparator itself should remain within a predetermined range.
The comparator (voltage comparator) is mainly constructed of a differential amplifier.
A differential amplifier (differential circuit) is designed so as to balance current capacities on the right and left. However, an offset (characteristic variation of transistors forming a differential pair) is normally produced due to variations in size, etc. of transistors that actually make up the differential amplification circuit.
The amount of offset of the comparator varies considerably.
An investigation conducted by the inventor of the present invention shows that the amount of offset sometimes goes beyond its allowable range a great deal.
Furthermore, it has been confirmed that in the case where the input/output offset of D/A converters is adjusted using a comparator whose offset exceeds such an allowable range, that is, in the case where the offset of the D/A converters is canceled by feedback control using a low accuracy comparator, an offset of 20 mV or more would still remain, which should originally remain within several mv.
To solve such a problem, it is necessary to cancel the offset (offset between right and left current capacities) of the comparator used for that purpose before canceling the input/output offset of the D/A converters.
However, it is actually difficult to provide a dedicated circuit only for offset cancellation of the comparator from the standpoint of preventing an expansion of the area occupied by a semiconductor apparatus (IC) or from the standpoint of a demand for low power consumption, etc.